The use of a separator between an anode and cathode in batteries, fuel cells, and electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemistry within the cell. Particularly, for example, in diaphragm chlorine generating cells, such a separator functions to restrain back migration of OH.sup.- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH.sup.- back migration has been found to significantly decrease current inefficiencies associated with a reaction of the OH.sup.- radical at the anode releasing oxygen.
More recently separators based upon an ion exchange copolymer have found increasing application in batteries, fuel cells, and electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name NAFION.RTM..
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising one to eight carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F, particularly where the --SO.sub.2 F group exists as the --(--SO.sub.2 NH)mQ form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers made from perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
Many chlorine cells use a sodium chloride brine feedstock. One drawback to the use in such cells of perfluorocarbon separators having pendant sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic formed in these cells, including OH.sup.- radicals, from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carbonyl groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes remotely positioned on opposite sides of the cell separator.
Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. Typical sandwich SPE cells are described in U.S. Pat. Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409.
At least one difficulty has surfaced in the preparation of SPE sandwiches employing reticulate electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a perfluorocarbon copolymeric membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the obverse membrane surface. To avoid heat damage to the copolymeric membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. For reasons related to reticulate electrode structural configuration, such pressure is generally required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode. As yet, the solution coating of such electrodes with perfluorocarbon copolymer has not been feasible principally due to difficulties in developing a suitable solvent for perfluorocarbon copolymer.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of at least partially solvated perfluorocarbon dispersions and for solution coating electrodes with the copolymer perfluorocarbon where the perfluorocarbon is of a relatively elevated equivalent weight desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing and/or solvating these higher equivalent weight perfluorocarbons.